The 6CY5  (P/N 154-0207-00) is a tetrode vacuum tube used in the 580 series.

Links

Some instruments using part 6CY5

Instrument Manufacturer Class Model Description Introduced
581 Tektronix Oscilloscope 581 100 (95) MHz oscilloscope 1959
581A Tektronix Oscilloscope 581A 85 MHz oscilloscope 1963
585 Tektronix Oscilloscope 585 100 (95) MHz oscilloscope 1959
585A Tektronix Oscilloscope 585 85 MHz oscilloscope 1963